Density Changes During Undrained Loading—Membrane Compliance

Abstract

Membrane compliance in the triaxial test may cause water‐content redistribution and an increase in specimen density during undrained cyclic loading. A reduction in effective confining pressure causes the membrane to rebound from penetration sites and causes pore fluid to migrate to the specimen periphery. As pore fluid is drawn from interior to peripheral voids, the skeletal grain structure contracts to balance the fluid volume. Thus, specimen density at the end of undrained cyclic loading may be considerably higher than the density at the start, even though the test is undrained. Properties measured at the end of the test may not represent actual in situ performance, as the specimen contracted due to membrane compliance. The final specimen density may be determined by measuring volume changes in drained hydrostatic load and unload tests. Volume changes due to membrane penetration may be used to compute changes in density during undrained loading. The basis for this determination is presented in this paper. Results are presented for unsluiced and sluiced gravel specimens.